The present invention relates to an apparatus of the type referred to in the art as a frontofocometer, designed for measuring the frontal power of ophthalmic lenses and more particularly of "soft" corneal contact lenses, i.e. contact lenses made from a somewhat hydrophilic material having a predetermined index of refraction.
In the present specification the term "somewhat hydrophilic material" is used to designate not only hydrophilic materials such as ethylene-glycol monomethacrylate copolymers and ethylene-glycol dimethacrylate copolymers, but also materials which, while not being themselves of a hydrophilic nature, such as silicones, have either undergone a proper treatment for imparting hydrophilic properties thereto, at least to their surfaces, or been coated with a surface layer of hydrophilic material.
It is known that contact lenses of hydrophilic material or of a somewhat hydrophilic nature are preserved, outside the periods in which they are worn by the user, in a special liquid, more particularly a physiological serum or salt solution having properties akeen to those of lachrymal liquor. This physiological salt solution keeps the contact lenses in a "swollen" condition and it is clear that the frontal power of a contact lens made of somewhat hydrophilic material varies according as it is swollen or not with physiological salt solution, or, in actual service, with a mixture of this physiological salt solution with lachrymal liquor. Under these conditions, it is clear that when it is desired to measure the frontal power of a contact lens of this character the measurement should be made when the lens conditions approximate as much as possible those obtaining when the lenses are worn by the user, i.e. in the swollen state, by using a liquid having an index of refraction very close to that of the lachrymal liquor, for example the physiological salt solution in which the lenses are normally stored.